The invention relates, generally, to power control in radio transmission systems.
FIG. 1 shows a cellular radio communication system. The system 100 includes cells 101-107. Each cell 101-107 includes a radio frequency transmitting and receiving base station 111-117 that allows downlink (base station to mobile unit) and uplink (mobile unit to base station) channels to be established. The uplink and downlink channels provide paths for communication between each mobile unit 120 and an associated base station 111-117. The system 100 may be implemented in accordance with Telecommunications Industry Association standard TIA/EIA IS-95 Mobile Station-Base Station Compatibility Standar ds for Dual-Mode Wideband Spread Spectrum Cellular System ("IS-95") or in accordance with the Japanese Association of Radio Industries and Businesses (ARIB) standard Specification of Mobiles station for 3G Mobile Station System Version 1.0, (3G CDMA) or other wideband CDMA (WCDMA) or multi-carrier CDMA (MC-CDMA) standard. An IS-95 CDMA, 3G CDMA, WCDMA, or MC-CDMA transmission system generally use code division multiple access (CDMA) protocols and quadrature phase shift keying (QPSK) modulation to establish a communication channel between a base station and a mobile receiver.
In a CDMA system, multiple uplink CDMA channels can share the same frequency spectrum. Similarly, multiple downlink channels can share another frequency spectrum. To enable this frequency sharing, each channel's baseband data signal is multiplied by a spreading code which uniquely spreads the channel's data over the entire uplink or downlink transmission spectrum. At a receiver, the unique spreading code assigned to a particular channel is used to despread a received signal and discriminate the desired channel from others occupying the same frequency spectrum. The technique may also be referred to as "spread spectrum" communication.
The use of the same frequency spectrum by multiple CDMA mobile units can increase the bandwidth efficiency of a communications system. However, this spectrum sharing also causes a gradual degradation of the performance of the system as the number of users increases. This gradual degradation occurs because mobile unit receiver s detect signals from other mobile units as noise. In general, the stronger the signal from a mobile unit, the more interference the base station experiences when receiving and demodulating signals from other mobile units. Ultimately, the power from one mobile unit may be great enough to terminate communications of other mobile units. Accordingly, it is extremely important in wireless CDMA communication systems to control the transmission power of all mobile units. In general, transmission power must be controlled both when a mobile unit initiates a channel to a base station, and for the duration of that channel's use.
In an IS-95 implementation, when a mobile unit initiates a CDMA channel with a base station, the mobile unit has not yet received power control information from the base station and, therefore, does not know the power level needed for effective transmission. If the mobile unit begins transmitting at a power level that is too high, it may interfere with the communications of other mobile units and may even terminate the communications of other mobile units. If the initial transmission power level is too low, the mobile unit will not be detected by the base station and a communication link will not be established. To resolve this issue, IS-95 includes a start up protocol whereby a mobile unit begins transmission at a low power level and gradually increases power until a channel is established with the base station.
After the channel has been established, the strength of a radio signal received over that channel may vary due to fades and reinforcements. Channel fading can cause degradation of the received signal and loss of the connection. Reinforcements may cause interference with other transmitters and/or receivers in the mobile radio system. Because of these time varying changes in the radio propagation environment, a mobile unit will need to adjust its transmission power during the use of a channel. To regulate power during use of a channel, a closed-loop power control system can be used. In a closed-loop system, a base station periodically sends power control messages to a mobile unit instructing the mobile unit to increase or decrease its transmission power level. In an IS-95 implementation, the power control messages are sent as a series of power control bits in an overhead portion of a downlink communications channel.
Delay exists between the transmission of a signal from a mobile unit on an uplink channel, the calculation of power control information from that uplink signal at a base station, and the receipt and processing of that power control information back at the mobile unit. In a rapidly varying propagation environment, this delay may result in unsuitable power control information being received and acted upon by the mobile unit. As a result, performance of the mobile unit and/or radio system may degrade. Consequently, improved means of setting a mobile unit's transmission power are desired.